Wire rods having superior strength and ductility for drawing and method for manufacturing the same

ABSTRACT

There are provided a wire rod for drawing having superior strength and ductility and a method for manufacturing the same. The wire rod for drawing comprises, by weight: carbon (C): 0.87 to 1.0%, manganese (Mn): 0.1 to 0.60%, silicon (Si): 0.3 to 1.0%, sulfur (S): 0.010% or less (excluding 0%), phosphorus (P): 0.011% or less (excluding 0%), chromium (Cr): 0.1 to 0.5%, nitrogen (N): 0.007% or less (excluding 0%), and the balance of iron (Fe) and other inevitable impurities, wherein the sum of the Si and Cr contents satisfies the following equation: 0.6≦Si+Cr≦1.2 (the contents of Si and Cr is represented by ‘% by weight’), and the wire rod has a pearlite structure.

TECHNICAL FIELD

The present invention relates to a wire rod for drawing having superior strength and ductility, the wire rod being used for a tire cord, a wire rope, a piano wire, a bridge steel wire and the like, and a method for manufacturing the same, and more particularly, to a wire rod for drawing having high strength and high ductility by controlling a C content to a suitable range and simultaneously adding Si and Cr together to produce a lamella structure of pearlite into a fine pearlite microstructure, and a method for manufacturing the same.

BACKGROUND ART

In general, there are three known methods for manufacturing a high-strength wire rod for drawing, as follows.

First, the strength of a base steel may be enhanced by adding a large amount of a strengthening element to the base steel. Carbon (C) is a representative example of the strengthening element. The strength of a desired wire rod is increasingly enhanced as the content of C increases from a hypoeutectoid zone to a eutectoid zone, and from a eutectoid zone to a hypereutectoid zone.

The increase in the C content results in an increase in fraction of hard cementite inside the wire rod, and allows a lamellar spacing of the pearlite structure to get compact, which leads to the improved strength of a steel material.

Second, a wire rod for drawing is prepared by drawing and heat-treating a rolled wire rod and finally processing the rolled wire rod into a wire rod. In this case, the rolled wire rod may be cured to drastically improve its strength. Since a lamellar spacing of the pearlite structure becomes fine, a strain-hardening coefficient increases, and a potential is piled up in the process of the wire rod, it is possible to cure the wire rod.

Third, strength of a wire rod may be enhanced by increasing a wire drawing strain of material, regardless of the above-mentioned processes. Here, the wire drawing strain of material is closely associated with the ductility of material. Where a material is not disconnected while being drawn, the steel material may be easily processed and the strength of a wire rod may be favorably improved.

However, the above-mentioned processes are not carried out in an independent manner, but associated with each other, such that they cause the changes in strength of the wire rod. Therefore, the above-mentioned processes have their limits on improvement of the strength since parameters of the respective processes are under the independent control.

Also, when alloy elements are simply added at a large amount in order to enhance strength of a wire rod, the ductility of the wire rod may be poor, and thus be disconnected when it is manufactured after the rolling process. Also, the increases in the C content may lead to the improved strength of the wire rod, but rather result in the deteriorated ductility of the wire rod.

DISCLOSURE OF INVENTION Technical Problem

The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a wire rod for drawing having high strength and high ductility by controlling a C content to a suitable range and simultaneously adding Si and Cr together to produce a lamella structure of pearlite into a fine pearlite microstructure.

Also, it is another object of the present invention to provide a method for manufacturing wire rod for drawing having high strength and high ductility.

Technical Solution

According to an aspect of the present invention, there is provided a wire rod for drawing which has superior strength and ductility, including, by weight: carbon (C): 0.87 to 1.0%, manganese (Mn): 0.1 to 0.60%, silicon (Si): 0.3 to 1.0%, sulfur (S): 0.010% or less (excluding 0%), phosphorus (P): 0.011% or less (excluding 0%), chromium (Cr): 0.1 to 0.5%, nitrogen (N): 0.007% or less (excluding 0%), and the balance of iron (Fe) and other inevitable impurities, wherein the sum (% by weight) of the Si and Cr contents satisfies the following equation: 0.6≦Si+Cr≦1.2, and the wire rod has a pearlite structure.

ADVANTAGEOUS EFFECTS

As described above, one exemplary embodiment of the present invention may provide a wire rod for drawing, which has high strength and high ductility, by controlling a C content to a suitable range and simultaneously adding Si and Cr together.

Also, another exemplary embodiment of the present invention may provide a method for manufacturing a wire rod for drawing having high strength and high ductility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating tensile strength and reduction of area of a wire rod for drawing according to the content of carbon (C).

FIG. 2 is a graph illustrating tensile strength and reduction of area of a wire rod for drawing according to the compositional ranges of components in the wire rod.

BEST MODE FOR CARRYING OUT THE INVENTION

To enhance the strength of conventional wire rods for drawing, a large amount of carbon is usually added to a steel sheet. From this fact, the present inventors have come to the following conclusions by thoroughly examining the relation between the content of carbon and the strength of the wire rod for drawing.

The strength of a wire rod increases as a content of carbon (C) increases from a hypoeutectoid zone to a hypereutectoid zone. FIG. 1 is a graph illustrating tensile strength and reduction of area of a wire rod according to the content of carbon (C). Referring to FIG. 1, when the content of C increase to a level of constant C content, it is difficult to expect improvement of the strength of the wire rod, and the strength of the wire rod is not enhanced, or rather decreased due to the small reduction of area.

Therefore, the strength and ductility of the wire rod for drawing may be secured by adjusting the C content to a content level at which the reduction of area of the wire rod may be secured without the continuous increases in the C content, and simultaneously adding other alloy elements, particularly Si and Cr, to produce a lamella structure of pearlite into a fine pearlite microstructure.

Hereinafter, compositional ranges of components in the steel sheet according to one exemplary embodiment of the present invention are described in more detail. In this entire specification, the term ‘percentage (%)’ used in the exemplary embodiments represents ‘% by weight’, unless indicated otherwise.

Carbon (C): 0.87 to 1.0%

Carbon (C) is a core element to secure strength of steel. In this case, when a content of C exceeds 1.0%, the reduction of area (RA) of the steel is decreased, which makes it impossible to expect the improvement of the strength of steel by a drawing process, whereas, when the content of C is less than 0.87%, it is difficult to secure a desired strength of steel. Therefore, it is desirable to define the C content to 0.87 to 1.0%.

Manganese (Mn): 0.1 to 0.6%

Manganese (Mn) is an element that is effective at enhancing hardenability of steel but causes severe center segregation. In this case, when a content of Mn exceeds 0.6%, Mn has high possibility to induce formation of a low-temperature structure. On the contrary, when the content of Mn is less than 0.1%, an addition effect of Mn may not be shown sufficiently. Therefore, it is desirable to define the Mn content to 0.1 to 0.6%.

Silicon (Si): 0.3 to 1.0%

In addition to the component Cr, silicon (Si) is an element that plays a very important role in the present invention. C functions to enhance the strength of steel with its increasing content, but results in the decrease in reduction of area of steel, which appears as limits on the improvement of the strength of steel. Also, C functions to precipitate coarse proeutectoid cementite beyond a hypereutectoid compositional range, which provides a main crack initiation position during a drawing process. The addition of Si does not facilitate the formation of proeutectoid cementite within the hypereutectoid compositional range, but functions to enhance the strength of steel by means of the solution strengthening.

Since Si is used as a deoxidizing agent in a steel-making process, a trace of Si is included in steel. When Si is added at a content of less than 0.3%, the strength and ductility of steel are not improved effectively. However, when the content of the added Si exceeds 1.0%, the ductility of lamellar ferrite may be deteriorated severely, thereby degrading wire drawability. Therefore, it is desirable to define the Si content to 0.3 to 1.0%.

Chromium (Cr): 0.1 to 0.5%

In addition to the component Si, chromium (Cr) is an element that plays a very important role in the present invention. Cr functions to improve the strength and ductility of steel by producing a lamella structure of pearlite into a fine pearlite microstructure. When a content of Cr is less than 0.1%, the lamellar structure of pearlite into a fine pearlite microstructure is not achieved sufficiently, whereas, when the content of Cr exceeds 0.5%, a pearlite transformation rate at constant temperature is slow, which adversely affects productivity of steel. Therefore, it is desirable to define the Cr content to 0.1 to 0.5%.

Silicon (Si)+Chromium (Cr): 0.6 to 1.2%

It is effective to add the components Si and Cr together. Here, when the sum of weights of the added components Si and Cr is in a range of 0.6 to 1.2%, the strength and ductility of steel are improved. When a content of Si+Cr is less than 0.6%, the strength of steel is not highly improved, whereas, when the content of Si+Cr exceeds 1.2%, the ductility of steel may be deteriorated. Therefore, it is desirable to define the Si+Cr content to 0.6 to 1.2%.

Sulfur (S): 0.010% or less (excluding 0%), Phosphorus (P): 0.011% or less (excluding 0%) and Nitrogen (N): 0.007% or less (excluding 0%)

Sulfur (S), Phosphorus (P) and Nitrogen (N) are impurities that are present in the manufacture of a wire rod. Large amounts of the impurities result in embrittlement of steel material, thereby causing disconnection of a wire rod during a drawing process. Therefore, the contents of the impurities are limited to upper limits of 0.010%, 0.011% and 0.007%, respectively.

Furthermore, the wire rod satisfying the requirements regarding the above-mentioned compositional ranges may further include nickel (Ni). Ni functions to improve the strength and ductility of a wire rod since it facilitates the plastic deformation of cementite during a drawing process by driving one more slip system of cementite. When a content of Ni is less than 0.3%, the strength and ductility of a wire rod are not significantly changed when compared to those of the wire rod that does not include Ni but satisfies the requirements regarding the above-mentioned compositional ranges. Therefore, it is desirable for the wire rod to include 0.3% or more of Ni. On the contrary, when the content of Ni exceeds 1.0%, an addition effect of expensive Ni on the improvement of the strength and ductility is not conspicuous, and thus uneconomical. Therefore, Ni may be more preferably used at a content of 0.3 to 1.0%.

In addition to the above-mentioned components, the wire rod for drawing according to one exemplary embodiment of the present invention includes the balance of iron (Fe) and other inevitable impurities.

The wire rod having the above-mentioned compositional ranges has a tensile strength of 1300 MPa or more and a 30% or more reduction of area.

Hereinafter, the structure of the wire rod according to one exemplary embodiment of the present invention is described in more detail.

For the wire rod having the above-mentioned compositional ranges, a pearlite structure has an interlamellar spacing of 130 nm or less.

After the wire rod is subject to a lead patenting process (LP, heat treatment prior to drawing), the pearlite structure of the wire rod has an interlamellar spacing of 50 nm or less. The finer the interlamellar spacing of the pearlite structure is, the higher the strength of the wire rod is.

Hereinafter, the method for manufacturing a wire rod for drawing according to one exemplary embodiment of the present invention is described in more detail.

The method for manufacturing a wire rod for drawing according to one exemplary embodiment of the present invention includes: heating a wire rod at a temperature of 1100 to 1300° C. in order to homogenize a wire rod and secure a hot rolling temperature of the wire rod, wherein the wire rod includes, by weight: carbon (C): 0.87 to 1.0%, manganese (Mn): 0.1 to 0.60%, silicon (Si): 0.3 to 1.0%, sulfur (S): 0.010% or less (excluding 0%), phosphorus (P): 0.011% or less (excluding 0%), chromium (Cr): 0.1 to 0.5%, nitrogen (N): 0.007% or less (excluding 0%), and the balance of iron (Fe) and other inevitable impurities, wherein the sum (% by weight) of the Si and Cr contents satisfies the following equation: 0.6=Si+Cr=1.2 (the contents of Si and Cr is represented by ‘% by weight’); hot-rolling the heated wire rod; and cooling the hot-rolled wire rod at a rate of 10 to 20° C./s in order to obtain a fine and uniform pearlite structure.

MODE FOR THE INVENTION

Hereinafter, exemplary embodiments of the present invention are described in more detail.

Example 1

Each of steel billets having components and their contents as listed in the following Table 1 was heated at 1100 to 1300° C., and hot-rolled, and then cooled at a rate of 10 to 20° C./s to obtain a wire rod. Then, each of the prepared wire rods was measured for tensile strength (TS), reduction of area (RA), and interlamellar spacing of a pearlite structure.

As listed in the following Table 1, it was revealed that wire rods of Comparative steels 1 to 6 have a tensile strength of 1119 to 1249 MPa, and show their 30% or less reduction of area, except for the Comparative steel 1. In the case of the Comparative steel 1, it was seen that the wire rod has a high reduction of area owing to the low C content (i.e. 0.82% by weight), but is not suitable for a high-strength steel due to the very low strength (i.e. 1119 MPa) of steel.

On the contrary, it was revealed that Inventive steels 1 to 5 have a tensile strength of 1300 MPa or more, and also show their 30% or more reduction of area. Comparing the Inventive steel 1 with the Comparative steel 4, it was revealed that the tensile strength of the Inventive steel is increased by 121 MPa with the increased Si content, and its reduction of area is also increased by 6.6%. FIG. 2 is a graph illustrating tensile strength and reduction of area of a wire rod for drawing according to the addition of Cr and Si in addition to 0.92% by weight of C. In FIG. 2, the rightmost bar graph represents a tensile strength and a reduction of area of the Inventive steel 1.

It was revealed that the strength of the Inventive steels 1 to 3 are enhanced without the big loss in the reduction of area as the Si content in the Inventive steels 1 to 3 increases. However, it was revealed that the strength of the Comparative steel 7 is enhanced by the addition of 1.512% by weight of Si, but its reduction of area is suddenly reduced to 19.3% when C is added at a content of greater than 1.0% by weight.

Also, it was revealed that, when Cr is added at a content of 0.496% by weight, the Inventive steel 4 shows its superior strength and ductility, i.e. a tensile strength of 1364 MPa and a 38.7% reduction of area. Also, it was seen that the Inventive steel 4 has a tensile strength of 1300 MP or more and a 30% or more reduction of area when the sum of the Si and Cr contents is in a range of 0.6 to 1.2% by weight. Also, it was revealed that the Inventive steel 5 shows its superior tensile strength and reduction of area when Ni is added at a content of 0.5% by weight.

For the wire rods of the Inventive steels, it is characterized in that their pearlite structures have an interlamellar spacing of 130 nm or less, and their superior strength and reduction of area are due to the fine interlamellar spacing.

TABLE 1 Interlamel- lar spacing(n Components (% by weight) TS RA m)spacing Kinds C Mn Si Cr Ni Si + Cr (MPa) (%) (nm) Inventive 0.92 0.297 0.513 0.200 — 0.713 1361 35.8 124 steel 1 Inventive 0.97 0.294 0.513 0.200 — 0.713 1385 32.4 128 steel 2 Inventive 0.92 0.294 0.995 0.198 — 1.193 1464 36.2 88 steel 3 Inventive 0.96 0.296 0.304 0.496 — 0.800 1364 38.7 127 steel 4 Inventive 0.96 0.297 0.300 0.301 0.500 0.601 1328 32.3 122 steel 5 Comp. 0.82 0.296 0.180 — — 0.180 1119 35.6 132 steel 1 Comp. 0.93 0.328 0.204 — — 0.204 1208 29.5 157 steel 2 Comp. 0.98 0.297 0.214 — — 0.214 1216 25.0 142 steel 3 Comp. 0.93 0.298 0.202 0.198 — 0.400 1240 29.2 139 steel 4 Comp. 0.97 0.297 0.200 0.200 — 0.400 1249 29.1 145 steel 5 Comp. 0.98 0.294 0.202 0.102 — 0.304 1219 25.6 134 steel 6 Comp. 1.02 0.297 1.512 0.198 — 1.710 1512 19.3 132 steel 7

Example 2

Each of the wire rods (Inventive steel 1, Comparative steels 4 and 5) prepared in the method of Example 1 was austenized at 1050° C., lead-patented at a solder pot temperature of 550° C. to obtain a steel sheet. Then, each of the steel sheets was measured for tensile strength and interlamellar spacing. The results are listed in the following Table 2.

As listed in Table 2, it was revealed that a tensile strength of the Inventive steel 1 is increased by 88 MPa due to the increased Si content, compared to the Comparative steel 4. Also, it was revealed that the Inventive steel 1 has a superior tensile strength to the Comparative steel 5 whose C content is higher than that of the Inventive steel 1. And it was seen that, owing to the combined addition of Si and Cr, the Inventive steel 1 shows its superior tensile strength even after the LP process (i.e. heat treatment). In this case, it was seen that the Inventive steel 1 has an interlamellar spacing of 26 nm, which is about a half the interlamellar spacings of the Comparative steels. This is why the addition of the component Si results in the increase in a eutectic temperature, and therefore a nucleation rate is accelerated with an increase in a supercooling degree.

TABLE 2 Tensile Interlamel- Components (% by weight) strength larspacing(nm) Kinds C Mn Si Cr Si + Cr (MPa) (nm) Inventive 0.92 0.297 0.513 0.200 0.713 1483 26 steel 1 Comp. 0.93 0.298 0.202 0.198 0.400 1395 51 steel 4 Comp. 0.97 0.297 0.200 0.200 0.400 1403 54 steel 5

Example 3

Each of the wire rods (Inventive steel 1, Comparative steels 4 and 5) prepared in the method of Examples 1 and 2 was drawn to obtain a steel sheet. Then, each of the steel sheets was measured for physical properties. The results are listed in the following Table 3. The drawing process was carried out at a constant strain of 3.2% or more, and a diameter of the final steel wire was 2.7 mm.

It was revealed that the Inventive steel 1 has all high physical properties such as tensile strength, twists and fatigue property, compared to those of the Comparative steels 4 and 5.

The twists represent the workability or ductility of a steel wire at maintaining its superior strength. Here, it was revealed that the Inventive steels have the superior physical properties to the Comparative steels. The superior ductility of a wire rod functions to reduce a disconnection rate of the wire rod during the drawing process and suppress the delamination in the wire rod.

Also, the fatigue property represents the increase in service life and durability of the wire rod. Here, it was revealed that the fatigue property is two times higher in the Inventive steels than the Comparative steels. Therefore, it was seen that, owing to the combined addition of Si and Cr, the Inventive steel 1 shows its superior ductility and fatigue property as well as the tensile strength even after the drawing process.

TABLE 3 Tensile strength Twists Hunter fatigue Kinds (MPa) (No.) (cycles) Inventive steel 1 3870 52 119,270 Comp. steel 4 3830 45 72,000 Comp. steel 5 3860 44 66,000 

1. A wire rod for drawing which has superior strength and ductility, comprising, by weight: carbon (C): 0.87 to 1.0%, manganese (Mn): 0.1 to 0.60%, silicon (Si): 0.3 to 1.0%, sulfur (S): 0.010% or less (excluding 0%), phosphorus (P): 0.011% or less (excluding 0%), chromium (Cr): 0.1 to 0.5%, nitrogen (N): 0.007% or less (excluding 0%). and the balance of iron (Fe) and other inevitable impurities, wherein the sum (% by weight) of the Si and Cr contents satisfies the following equation: 0.6≦Si+Cr≦1.2, and the wire rod has a pearlite structure.
 2. The wire rod for drawing of claim 1, further comprising 0.3% by weight or more of nickel (Ni).
 3. The wire rod for drawing of claim 1, wherein the wire rod has a tensile strength of 1300 MPa or more, and a 30% or more reduction of area.
 4. The wire rod for drawing of claim 1, wherein the pearlite structure of the wire rod has an interlamellar spacing of 130 nm or less.
 5. The wire rod for drawing of claim 1, wherein, after the wire rod is subject to a lead patenting process (LP, heat treatment prior to drawing), the pearlite structure of the wire rod has an interlamellar spacing of 50 nm or less.
 6. The wire rod for drawing of claim 1, wherein, after the wire rod is subject to a drawing process, the wire rod has 50 twists or more.
 7. A method for manufacturing a wire rod for drawing having superior strength and ductility, comprising: heating a wire rod at a temperature of 1100 to 1300° C., wherein the wire rod comprises, by weight: carbon (C): 0.87 to 1.0%, manganese (Mn): 0.1 to 0.60%, silicon (Si): 0.3 to 1.0%, sulfur (S): 0.010% or less (excluding 0%), phosphorus (P): 0.011% or less (excluding 0%), chromium (Cr): 0.1 to 0.5%, nitrogen (N): 0.007% or less (excluding 0%), and the balance of iron (Fe) and other inevitable impurities, wherein the sum (% by weight) of the Si and Cr contents satisfies the following equation: 0.6≦Si+Cr≦1.2, and the wire rod has a pearlite structure; hot-rolling the heated wire rod; and cooling the hot-rolled wire rod at a rate of 10 to 20° C./s.
 8. The wire rod for drawing of claim 2, wherein, after the wire rod is subject to a drawing process, the wire rod has 50 twists or more.
 9. The wire rod for drawing of claim 3, wherein, after the wire rod is subject to a drawing process, the wire rod has 50 twists or more.
 10. The wire rod for drawing of claim 4, wherein, after the wire rod is subject to a drawing process, the wire rod has 50 twists or more.
 11. The wire rod for drawing of claim 5, wherein, after the wire rod is subject to a drawing process, the wire rod has 50 twists or more. 